1. Field of Invention
The present invention relates to a process and apparatus for coupling laser pulses and other light beams into an optical carrier. The present invention enables convenient and cost-effective delivery of laser pulses and light signals via an optical carrier.
2. Discussion of the Background
In conventional applications of optical fiber, a continuous wave (CW) laser output is coupled into an optical fiber and propagated to a remote site. Optical damage to a fiber tip of the optical fiber by the laser output power is seldom a problem in many applications due to the relatively low power density input at the entry surface to the fiber tip. However, as the laser power density increases, for example in a pulse laser, the peak power density of the laser pulse can easily exceed a surface or bulk damage threshold of the fiber material. For a fiber material made of silica, the surface damage threshold (˜100 MW/cm2) as well as the bulk damage threshold (˜200 MW/cm2) can be exceeded. Previous work on optical damage using ultraviolet (UV) and infrared (IR) lasers has been described in F. Rainer et al., “Laser damage threshold at short wavelength,” Applied Optics, Vol. 21, No. 10, pp. 1722-1724, May 1982, and in Wuthrich et al., “Optical Damage thresholds at 2.94 μm in fluoride glass fibers” Applied Optics, Vol. 31, No. 27, pp. 5833-5837, September 1992, the entire contents of which are incorporated herein by reference.
FIG. 1A is an exemplary schematic of a convergent-coupling configuration conventionally used to couple, by a focussing lens 1, a laser beam 3 onto a focal spot 4 of an optical fiber 2. In the convergent-coupling geometry, power densities can exceed the damage thresholds of the optical carrier materials (e.g. optical fibers and waveguides). For example, the peak power density of a focussed YAG laser beam is about 26 GW/cm2, considering the pulse energy of 10 μJ, the 10 nsec pulse-width, and the diffraction limited spot diameter of ˜1 μm. This power density is far beyond the surface (or bulk) damage threshold of the silica in a conventional optical fiber. While the laser beam can be matched to an aperture and focussed, as shown in FIG. 1B, into the bulk of the optical fiber, the bulk material damage threshold is usually only a few times (e.g., 2-3 times) larger than the surface damage threshold, providing only a marginal increase in a safe power density margin. If beam alignment (or in this case misalignment) is such that an internal focus of the beam is on a wall of the optical carrier, then the intense power density will start to damage the optical carrier wall surface. Once initiated, the damage will gradually propagate to the center of the optical fiber. Thus, optical damage regardless of the point of focus in convergent-coupling is always a concern at the high power densities of pulse lasers.
Similar problems do occur in hollow-waveguides, as described by Nubling et al., “Hollow-waveguide delivery systems for high power, industrial lasers”, Applied Optics, Vol. 34, No. 3, pp. 372-380, January 1996, the entire contents of which are incorporated herein by reference. While bulk damage in a hollow-waveguide is not a problem, in that the center of the hollow-waveguide is filled with air, the potential for damage on an internal surface of the hollow-waveguide due to misalignment of the laser beam is still a problem, similar to the afore-mentioned damage propagation problem.